Aiflow meter pot source - Bosch CIS
 

I have just ordered the feedback pot for my airflow meter ('91 190E 2.3) from a place that sells Bosch generic parts. A source was listed previously in Russia, but giving out my CC# to them was not for me. This location is in VA, and I'll report back how the transaction goes. Price is $71 with $9 shipping, and tax if in VA:

https://www.olyonline.com

BTW, the part# for mine is 3437 224 035, which is the same as someone listed for a W124 engine - but I don't recall which one.

Steve

The part just arrived tonight, and it appears identical - and does have the same part #. Two weeks for a special order Bosch part - not bad since the car has been working ok since, with the old one and some wiper-tweaking I did. I don't know when I'll install it, since I want plenty of time to get the new one as identical to the old as possible when I tighten the screws. Pics of old vs. new:

new:

here is the detail of the wiper arm that sweeps the pot elements:

and, old and new shots show the surprise hidden adjustment, presumably a trimpot, from the outside view:

Oops!

There are three adjustments on reassembly, which could make this a lot of fun:

-the hidden trimpot
-the whole body rotates about the wiper arm axis a few degrees
-there is an adjustment of some sort inside the wiper arm pivot, in the body of the airflow meter unit

Sbourg,

I appreciate the photos and the info. From what I`ve read, setting those replacement pots can be a pain.

Keep us posted and let us know the results after you swap it out.

Tinker

Not sure I wll be replacing this soon - sometime this summer, though, or immediately if problems return.

Note the exacto knife scribe mark I made on the body of the airflow meter BEFORE the first time I removed the pot. Very important to have a reference whether putting back the original or using the old one as a reference for the new.

Steve

Did you have to remove the intake-fuel distributor from the engine?

Your 6 may be different from my 4, but the only things I removed were the air cleaner housing, the bolt securing the pressure regulator, and the fuel connection from the regulator to the fuel distributor. Worst case, I could envision you might have to remove one or more of he injector lines in a 6.

The 4 screws retaining the pot are hidden under pry-off covers, then the pot just lifts out. Don't forget the scribe mark! Note the white streaks on the element of the old pot. This is where the wiper brush has worn thru the deposited resistance element to the ceramic substrate. No amount of cleaning or abrasion with eraser will replace the missing conductor. When the wiper brush hits the white spots, contact is opened.

My temp fix involved slightly bending the brush wires to the side to contact new material. This way, some of them are likely to retain contact with the unabraded surface. This is a very delicate and dicey procedure that I wouldn't recommend to anyone not willing to replace the whole airflow meter if it goes awry.

Steve

Please keep us abreast of any new developments with the installation of the poteniometer!

anyone know the part # for this part for the 86 190e 16v,
i think i need one, and that site only sells the whole pot for
600.00, in there search engine , i have a extra pot i would like to rebuild,
thanks
Dave

The part# is a Bosch one - not Mercedes - and is clearly marked on the outside. You can see the number for this one in the pics I posted. Do a search on that site for Bosch parts.

Steve

here is a excellent bosch site i found this afternoon,
http://www.boschusa.com/home.asp
it gives most of the MB bosch fuel injection part # for the 16v and the 8v,
The plus is that it tells you the closest place to you that can get
bosch parts, my local Acme auto, or pep boys told me 64.00 for the little black meter or 399.00 for the whole air flow meter new!! Alot better then the 650 they want at MB dealer with the club discount.

Sbourg,

The parts number you listed must be a MB number. Both the Olympic and Bosch sites list the part number as a 0 261 210 066?

And Olympic states a $166 price???

Tinker

The part# I listed is lifted right from the old part. It is visible in the photos. If you go to the site, select the 'Bosch' parts link, and enter the number without spaces, it will return the correct part, and the $71 price. I ordered that, and that's what they charged. The part that came is as pictured, and you can see it has the same part# as the old.

If you need this part, I suggest you get the original part# right off the old part. They will probably refer you to an updated# if the old part has been superceded, but this is always more reliable than searching by function name.

Steve

Update - flaky behavior today and yesterday convinced me to swap out the pot for the new when we got home. Examination showed geometry and alignment of the new pot identical to old, so adequate results could be expected by just aligning the new one to the scribe mark on the airflow meter body. The low-end setpoint pot read 57.7 Ohms on the new pot and 108 Ohms on the old, so I set the new one to match. Everything reassembled, operation seems normal, so I'll see how it drives headed to work tomorrow.

Steve

Drive into work this morning was 16 mi. of uneventful commute on local roads, no traffic. Idle always returned immediately to about 700 rpm, no hunting, no surging on deceleration. This is the most 'normal' it has ever been, and much better than recently with the old pot. Too soon to say whether problems which occured intrmittently are gone - I'll have to wait about a week or so to be sure.

Steve

thanks for the info Steve, I am going to order one this week and
install it too.
thanks agian
Dave

A week has passed without a hitch in operation of the airflow meter - never any decel surge, idle always stays at 700 warm, briefly at 1100 when engine started cold, then 700. Initial tank of gas shows improved mileage, too, but a few tankfuls before final judgement on that.

Steve

Steve,

Glad to hear of your successful results. So your symptoms were a hunting idle?

You mentioned the three adjustments. But it sounds like you scribbed a mark on the meter housing and matched. You then stated; "The low-end setpoint pot read 57.7 Ohms on the new pot and 108 Ohms on the old, so I set the new one to match" So the low-end setpoint was checked by resistance through the pot? And what/where to adjust to achieve the desired 108Ohms?

Boy I wish the 16V`s were as easy to replace as the 8v`s.

Thanks for the posts.

Tinker

Hunting idle, with occasional swings up to 2000 rpm or more and back, and surging on decel. According to Bosch and Stevebfl, if I follow, the pot is used to supply enrichment demand input to the ECU, and not to track the actual airflow value. That is accomplished mechanically by the airflow meter coupling to the fuel distributor plunger. I don't know what the cause of the above symptoms is, but my guess is that the pot feedback is erratic at constant no throttle, and the ECU interprets this as demand for enrichment, and compensates for the resulting rich signal from the Lambda sensor by opening the idle control valve. This IS just a guess - I don't know the actual ECU logic.

The scribe mark is visible on the pic of the airflow meter body with the pot removed. The pot housing is designed to rotate precisely around the axis of the wiper arm - visible in that pic. The mark allows the new pot to be indexed in rotation depending on the alignment of the internal circuit board relative to the one in the old pot - so the same features line up in precisely the same way. In fact, the old and new units sported identical internal alignment, so the new one could simply be lined up to the scribe mark the same way as the old.

Another mechanical adjustment may exist on the end of the wiper axis. If so, it would be redundant, and in any event I did not need to investigate it.

The circuit board has three solder tabs visible on one edge. Two of these connect to pins 2 and 3 of the outside connector. The third, at the 'low' end of the circuit board, connects thru the trimpot to connector pin 1. This puts the trimpot in series with the pot, and the trimpot value can be measured between the board solder tab and pin 1.

Since the resistance value of my new pot was within close tolerance the same as the value of my old one, adjusting the new trimpot to the same value as the old puts the voltage monitored at the low end of the pot the same as before. If you are substituting a pot of a different value, my guess would be to keep the ratio of the trimpot resistance to pot resistance the same as the old, and maybe it will work. The trimpot adjusting screw is visible on the outside of the (new) pot housing. It is covered on the old one.

Bottom line, though, is that replacing and adjusting the pot alone is far easier than it must be to replace the whole airflow meter - which would require the mechanical mixture adjustment monitoring EHA current AND somehow a calibration of the pot.

Steve

Steve,

congratulation and thank you for the write up.

When we replaced the pots (I organized a group buy for 22 potentiometers :) ) we adjusted the new ones to 0.7V between pin 1 and 2 with the engine idleing. That 0.7V is the test value from the workshop manual/CD.

Can you check the voltage?

Problem with Ohms is that you can't get a correct value from an old and used potentiometer...

Tinker,

the potentiometer for the 2.5-16 is the same as "allways"...
Replacing that is not much more work than on a 300E I think.

bis denn,
Christian

1989 300TE

Thanks for the spec, Chris - I don't have the CD yet, and my Haynes doesn't cover '89 on. Since part of my motivation for doing much of the work I am concentrating on at this time is an impending emission test requirement, I would prefer to have the adjustment 'dead-on' rather than 'very close'. Also glad to share my experiences - that is what this site is about for me, since I have learned so much from others sharing.

Steve

I plan to try this fix for my surging idle..
 

I have only a couple of things left to fix on my 89 W126 3.0. Inconsistent ( surging) idle is one of them.. The others are motor mounts, and both muflers..

Guys, I have a couple of questions I hope you can answer..

When I adjust the voltage to .7V between pins one and two, do I accomplish this by adjustment of the trim pot screw, or by swiveling the unit on its' axis????

Does anyone know if the part # on mine will be the same???

Steve, If I read correctly.. You simply removed the old unit, adjusted the trim pot screw to the same value, and re-installed in the exact same location.. With good results.. Correct????

THanks,
Jay

Jay, since this pot has a complex and non-linear taper, precise mechanical alignment is necessary to put the wiper on the correct spot for idle. This will give the correct rate-of-change as the wiper moves off-idle. Based on this, I assume the mechanical alignment should be independent of measured voltage, and the voltage offset at idle set electrically, with the trimpot, after mechanical alignment.

The procedure I used is as you describe, as are the results. As long as the new pot (measured at the board connection pads) is the same resistance as the old, the trim pot adjusted to the same resistance value should give a very accurate result. Note that wear on the pot trace should have little or no effect on the total board resistance, since the actual resistance element is much wider than the area the wiper sweeps. Additionally - on mine, at least - the worst wear is on the 'tracking' element, which does not affect a measurement of the resistance element. I do intend now to check my claim with a measurement - probably this weekend.

The Bosch part# is embossed on the outside of the pot, and I would read it off yours to ensure you order the correct part.

Steve

Thanks Steve.. On mine, making any adjustment after it's installed will be quite difficult, due to its' location, and the fact that some things are in the way..

I took down the part # last night, and I hope I read it wrong..

I have 3437224015, which is one digit off from what you have, and is not found in the bosh parts at that website.. I'll double check the # tonight...

Some searching answered my question.. the # ending in 35, is an updated part # an appears to be the correct one..

Jay,

3437224015 is the old number, this current part has a 035 at the end. That will do it :)

I really would adjust the pot to voltage, but: chacun a son goût!

Reason is, the resistor is weared, that is why you change the pot. You can messure the Ohms of the installed one - but how can you be shure it's the correct value? My pot showed chaotic values at or near idle position, every small movement of the AMS plate forced the needle of the Ohm meter to swing...

In the early KE instructions (for M102 in the W201) Mercedes advised to check the Ohms, in the later docs (current CD or manuals) they advice to check the voltage. It should be 0.7V +- 0.25V - so why not set the new pot to the requiered value?

If you do so, you will realize, that smallest movements of the pot will lead to relativly large changes in voltage - I don't think that an adjustment to a mark will be sufficient.

The adjustment should be done by rotating the pot. Carefully screw it down so that you can move it by small hits of a screw driver...

Installation is not that difficult but you have to disconnect the fuel pressure regulator to get access to the lower right screw.

Older pots are fixed by philips (spelling?) screws, newer ones have torx T15. You will need good tools.

hope that helps,
bis denn,
Christian

1989 300TE

Thanks Chris, and Steve.. I just ordered it, and I'll post my results in a couple of weeks..

In the mean time, I'm a little confused about what the trim pot adjustment screw does.. Is it factory adjusted and I don't need to mess with it???

I love this board:D If I had not found it, I never would have bought a Benz in the first place..

Chris, some questions and comments:

Wear on the pot should not affect the total resistance, since that is not affected by the wiper, and not noticeably by the wear marks either. I would expect more sample-to-sample variations than any effects due to wear. You are right - measuring the resistance of the wiper output on the old pot (pin 2 when installed) would be useless. This would be due both to wear, and due to the fact the ECU input is undoubtably very high impedance compared to the pot series resistance of the wiper - it is only concerned with the voltage, not source impedance variations.

I also feel that monitoring the output voltage in-situ is the most accurate way to set the mechanical alignment - but only if the trimpot is preset accurately. You don't mention the procedure for that - is there one? It was certainly set incorrectly on the new pot I received when compared to the old.

Rambling:

If I didn't preset the trimpot before installation, my monitored bottom-end voltage would be half the value it was previously. Still, we are only talking 1.4% vs 2.7% of full scale reading at a point lower than idle speed, so maybe MB doesn't think this adjustment matters. If the voltage at idle is supposed to be 0.7V, +/- 0.25V, this is an adjustment to an accuracy of 35% at a voltage level that is probably near 10% of full scale? It would seem that adjusting the trimpot would only have an effect on voltage that would be encountered during a backfire through the intake.

It will be interesting to see what voltage I measure - I will be surprised if my adjustment was wrong considering how carefully I measured. Of course, if the old pot had been moved in the vehicle's previous life, all bets would be off, but I don't think that was the case.

Steve

Steve,

>Wear on the pot should not affect the total resistance,

you are right, the resistance between pin 1 and 3 is constant at 4 kOhm (at all 23 pots i measured).
Maybe I didn't understand you completely.

Which resistance did you measure and to what did you adjust the thing?

1-3 is fixed at 4kOhm
1-2 varies first on the position of the MAS plate and second on the wear
so what? :confused:

>I also feel that monitoring the output voltage in-situ is the most accurate way to set the mechanical alignment - but only if the trimpot is preset accurately. You don't mention the procedure for that - is there one?

That is one of the great miracles :D
AFAIK the trim pot is in serie to the pin 1-2 resistor. Adjusting the trim pot may help you to get the correct value, if not possible by mechanical adjustment only.

Since Mercedes / the dealer doesn't change the pot but only the AMS unit , AFAIK there is no official instruction for adjustment of the pot. The new and complete unit comes adjusted from a flow bench but that will not help us, I think.

In other words, I aligned my pot mechanically to 0.7 V and did not touch the trim potentiometer.

>Still, we are only talking 1.4% vs 2.7% of full scale reading at a point lower than idle speed, so maybe MB doesn't think this adjustment matters.

As said before, only BOSCH knows how to adjust that thing, in case they still know... To be serious, they will know, since they still (produce and) sell MAS units as replacements.

>I will be surprised if my adjustment was wrong considering how carefully I measured.

Your alignment will be good or at least very close to spec. If you turn the potentiometer, the idle will change dramatically. Since you like the new life of your engine, the adjustment will be fine... :)

best regards,
Christian

1989 300TE

Chris,

"If you turn the potentiometer, the idle will change dramatically."

My understanding is that this pot does not affect idle speed nor baseline mixture. Are you saying that if you set the pot to a different position, the speed changes permanently, or just briefly?

"1-3 is fixed at 4kOhm
1-2 varies first on the position of the MAS plate and second on the wear"

The trimpot is actually in series with pin 1 and the board, i.e. pin 1 does not connect to the board as do 2 and 3. Therefore, 1-3 is actually not fixed - it varies as the trimpot is turned. Since the trimpot is at the end of the board closest to 'idle', it will affect the idle voltage more than a condition of high demand.

However, I measured the trimpot values directly, and in the case of my old board, it was adjusted to 108 Ohms, and in the new one to 57 Ohms. Since this affects the voltage bias at the 'bottom' end of the potentiometer, I adjusted the new the same as old. Note that if measuring 1-3 on the connector, this is a difference between 4057 and 4108 Ohms - not a lot.

I guess we still don't know the function of the trim pot, though it might be an alternative way to adjust idle voltage. In general, circuits provide two adjustments to track a range, and in this case that is provided via the rotation and the trimpot. It's only useful if there is a procedure for adjusting at two points, though, and it sounds as though Mercedes only provides one.

Steve

I would think that it would be very hard for the manufacturer
to hold a perfect resistance tolorence with a carbon/wiper style pot with mechanical indexing/rotational positioning..
So, they add the pot to trim each unit ...which would translate
to me that one would want to use the trim pot to adjust the output of the unit in real voltage reading specs [in this case .7v]

Steve,

>My understanding is that this pot does not affect idle speed nor baseline mixture.

It does - at least at the M103. Maybe the M102 is different?

>Are you saying that if you set the pot to a different position, the speed changes permanently, or just briefly?

Permanently - at least until I stoped it. I got an idle at 2500/min by just de-aligning the housing for some millimeters.

>The trimpot is actually in series with pin 1 and the board, i.e. pin 1 does not connect to the board as do 2 and 3. Therefore, 1-3 is actually not fixed - it varies as the trimpot is turned.

:)
Sorry, i forgot to add: "during normal operation" - ok?

>I guess we still don't know the function of the trim pot, though it might be an alternative way to adjust idle voltage.

No, we don't know yet.
Since the pot will be adjusted when mounted on the AMS housing on a flow bench, I think (guess) the procedure is as follows:

We are at the BOSCH factory that builds these AMS units. There are many mechanical parts inside the AMS, that are already assembled. Now the pot...

1st step - the pot is aligned mechanically at a given position and srewed down to theAMS housing. That is an assembling step during production.

2nd step - the AMS unit is mounted on the flow bench and the complete set is fine tuned so that at a given air flow a given resistance (or output voltage at a given input voltage) is reached. This will compensate all tolerances of the mechanical part (weigh of the sensor plate, spring inside the unit...)
Therefore the trim pot is used to reach the value of the test aparatus and afterwards the trim pot is sealed.
This is a calibration step during production.

Only guessing, but it sounds good to me. :p

What do you think?

bis denn,
Christian

1989 300TE

Arthur,

why can you express this in only a few words - I needed a lot more :)
Maybe I'm better if we try this in german? :rolleyes:

But we are assuming the same, right?

bis denn,
Christian

1989 300TE

Absolutely..
Each unit is calibrated on an individual unit basis with the pot --bringing everything [ tensions,mechanics, posistions, etc ] to a spec. output before it goes in the box...that way, each unit should perform the same as a replacement without having to adjust its calibration..Trim pots are resistance circuit calibrators used to zero in on a required /design specification ..
But , when you are getting inside these things [ which I love to do] , you now have to get it back to spec... so, you play around
with angles and trims til you fiqure it out...FUN and rewarding...
and a cheap education.....in both money savings and mechanics

This scenario sounds good to me - but I doubt then if any 'pre' calibration of the trimpot is done when the pot is bought as a separate unit. I.e, the value I measured on the new part is probably purely arbitrary. Also, since I am convinced my mechanical alignment on the flow meter body is pretty precise, it seems the logical - and easiest - way to cal the pot would be to fine-tune the trimpot to get the 0.7 Volts (or leave it if the reading is 0.45 to 0.95 Volts).

Still puzzled about the change in idle speed you noticed, Chris, if it didn't stabilize back down in a few moments. My understanding is the idle speed is controlled by rpm sensor, not any mixture adjustment. Although I would expect any perturbing of the mixture to cause a temporary speed instability, the pot adjustment does not cause a mechanical mistuning of the mixture that requires a correcting offset by the EHA. It is simply another input signal to the ECU, which could be ignored if it is overridden by the lambda sensor. Maybe it just takes longer to stabilize than you allowed? Anyway, another puzzle.

Steve

True
With any trim device , be it a mechanical [ an aircraft elevator trim comes to mind here] or electrical trim input,
one usually like to see the trim [ in this case . a pot] to be near that 1/2 travel point when the spec is obtained ..Reasoning here is that trims should have
a +/- adjustment where 1/2 way would be considered zero trim..
Guess we could call this Trim Center..This allows trimming in either direction and if a trim is exactly at zero to obtain the desired specs, all the better...

As it stands now.. I plan to place the new unit in the same position as the old.. Tighten it down.. Re-assemble everything. Start it up , and adjust to 0.7 volts or so with the trim pot. That just seems to make good sense.. That's my plan..

I'll post my results.. Thanks for all the great reading..

Jay

I will be interested to see what results you guys have trying to adjust these.
I turned the setscrew at least 20 turns with the engine running and a meter attached, and did not change the voltage at all. The only difference I could see was with key on/eng off, where clockwise turns raised the voltage. The reading I saw under these conditions was a fraction of running voltage. I finally stopped after raising voltage from about .017 to .2v. Running, on the other hand, was below .7v at idle both before and after adjustment attempts, and simply installing the pot solved both stalling and hunting to 2k rpm sysmtoms.

I wonder if this relates to what SteveB mentioned about .7v being the magic number to get below to send a signal to the ECU.

I had mostly put this issue on the shelf until I addressed some other problems and will pick it up again after I get the head back on (hopefully) this weekend.

On the other hand.. I may adjust the resistance of the trim pot to match the old one..Just install it, and if it runs good, solves both my idle surging, and hot start problems.. I may not mess with it??:D

I feel fairly confident that this is the cause of both problems..

Hey Guys,

I just got my potentiometer today for the 16v, I installed it ,
and set it .7, started the car and it idled badly(but steady for the first time in in some while) and when I gave it some revvvs it would stall every time coming back to idle. I measured it agian it was .7 still,
so , I losened the pot screws and started tuning by ear ,lol
I found a sweet spot(1.13) where it was running very well,
but the idle is a firm 1100 or 900 w/A/C, I coundn't get it to idle at 900 and not stall after revvving it, maybe if I adjust the mixture screw? or is this what the little trim screw is for???

All and all I am very pleased with the new pot, no more screwie up and down idle, no more hesitation, cold start
was finally normal, it seems faster through the gears

I really have to thank Steve (my newest hero) and everyone , I have replaced ALOT for parts (almost everthing ) to find these problems.
I had no idea about this most important CIS feature, and the best part about this is it a cheap ,quick fix.
I will spread the word
THANKS AGAIN
Dave
190e 2.3 16v

My final research on the topic. This morning I popped the cap on the pot connector and tested voltages. Listed in sequence 1,2,3, measured relative to a ground point on the case of the fuel distributor, 0.3% accurate meter:

-Ignition on, engine off:

0.011V, 0.142V, 4.90V

-Engine on idle down and relatively stable at 700rpm:

0.010V, 0.62V to 0.67V, 4.90V

Because of these readings, I did not try to either adjust the trimpot or the rotation of the potentiometer - the results of which could have been interesting.

My conclusion in this whole adventure is that the pot is a wear item, and should be eventually replaced in every KE Bosch system. Checking for the wear is pretty easy on most cars, with some care in alignment. Since the airflow meter body is adjustable for wear - and a POA to calibrate, not to mention hideously expensive - replacement of the pot alone is a much better option. Too bad MB doesn't supply the part.

Steve

Dave,

What year is your 16V? And what was the Bosch part number on your pot?

Did you remove the whole air flow meter to get to the pot?

Thanks.

Tinker

THe part # question, and year model were answered on page one, post one..

the removal process was answered in another thread..

http://www.peachparts.com/shopforum/tech-help/68427-help-pot-removal.html

If you have a part # ending in 15, it crosses over to the one ending in 35..

tinker

It is a 86 16v, the pot i took out was3-430-591-017, since bosch doesn't make this part anymore I ended up using pot
3-437-224-035, I didn't take off the air flow, I just
took off the cold start lines and the fuel reg lines so i could see it better, and removed the distributer cap , so my hand could fit
behind intake, . it took about me about 30min
I know the 87 16v had a differnent airflow then the 86, I
bet it was pot 035 is the only differance
Dave

Im sorry Tinker.. I didn't realize you were asking Dave specifically..

Please accept my appologies..

Jay

Inadvertant additional research. Filled up again today, and mileage definitely seems to have improved by about 2-3 mpg, and is more in the range I expected from this car with my driving habits.

Steve